Change parameter mark to standard in wp-org main theme

[tobifjellner]

Changeset https://meta.trac.wordpress.org/changeset/8042 changed so that the current number of plugins in the catalog will be used, instead of a hard-coded value. That's great.
But the style %1s where, for consistency, it is preferred to use %1$s

[dd32]

In 8046:

WordPress Main Theme: Switch to using the more standard %1$s format that's better known.

Missed in r8042.
Props tobifjellner for noticing.
See #3870.
Fixes #4050.